Fiddler crabs are small, semi-terrestrial crabs in the genus Uca that are distinctive in the extreme size difference between the left and right claws in males. They are most closely related to the ghost crabs (genus Ocypode). As of 2007, there were 97 recognized species/subspecies of fiddler crabs. Crane (1975) produced a landmark work on fiddler crab biology and systematics which is widely available online. Crane recognized 62 species, most of which are still recognized as valid, although many of her subspecies are now generally treated as full species and several additional species have been described subsequent to her publication. These taxonomic changes were detailed by Rosenberg (2001) and are summarized by him at http://www.fiddlercrab.info/uca_systematics.html.
All fiddler crabs are largely diurnal, active at low tide, and gregarious. They feed by filtering bits of organic matter from the shore surface. The small claw, which is specialized for lifting mud or sand, rhythmically brings pinches to the mouthparts. Here food is separated from the mineral matter which, accumulating outside, is periodically wiped off and dropped to form a growing line of pellets.
Male fiddler crabs wave their "major claw" (the large claw) to attract females and repel male rivals. Males may also produce sounds by stridulation and substrate-thumping and even build structures near their burrows to help attract mates. Mating generally occurs once the male has led the female into his burrow (Brusca and Brusca 2003). A variety of videos showing fiddler crabs feeding, displaying, and mating can be viewed at http://www.fiddlercrab.info/uca_video.html.
Fertilized female fiddler crabs carry hundreds or thousands of eggs beneath the abdomen (females in this condition are sometimes known as "sponge" crabs). When the eggs are ready to hatch, the female enters the water and the eggs release microscopic free-swimming larvae. These early stage larvae are known as zoeae or zoeas (singular: zoea). The larvae live in the open water as part of the plankton. As they grow, they pass through a number of molt stages. Older post-larval crabs are known as megalopa (plural: megalopae, megalopa, or megalopas). At the end of the final larval stage, the larvae molt into immature crabs. The amount of time spent as a swimming larva (i.e, from hatching to true crab stage) varies among species, but ranges from a few weeks to a few months. The crabs return to land and begin to grow. Juvenile male and female crabs look alike. As they grown larger and mature into adults, the secondary sexual characteristics (such as the asymmetric claws) begin to develop. Adult crabs mate and the cycle starts over again.
The approximate geographic distributions of the various fiddler crab species of the world (based largely on Crane 1975 but updated with new information up to around 2005) are available at http://www.fiddlercrab.info/uca_maps.html. A comprehensive list of fiddler crab literature references through around 2006 is available from http://www.fiddlercrab.info/.
Fiddler crabs have been the subject of many investigations of behavior, especially with respect to sexual selection and the function of the male's enlarged claw . Summaries of a sampling of such research can be found on the Science News website.
(Crane 1975; Rosenberg 2001; www.fiddlercrab.info)
Water temperature and chemistry ranges based on 2 samples.
Depth range (m): -4 - -4
Temperature range (°C): 27.858 - 27.858
Nitrate (umol/L): 0.255 - 0.255
Salinity (PPS): 33.450 - 33.450
Oxygen (ml/l): 4.615 - 4.615
Phosphate (umol/l): 0.162 - 0.162
Silicate (umol/l): 2.664 - 2.664
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.
Known prey organisms
Based on studies in:
USA: Georgia (Marine)
This list may not be complete but is based on published studies.
Based on studies in:
USA: Georgia (Marine)
This list may not be complete but is based on published studies.
Evolution and Systematics
The mouthparts of fiddler crabs filter fine materials from sediment using spoon-shaped setae (stiff hair-like structures) to hold sand grains, while brush-shaped setae brush off diatoms and bacteria for eating.
Fiddler crabs (Uca spp.), are deposit feeders, ingesting organic matter from exposed mud at low tide. "Sediment is sorted within the buccal cavity. The outermost mouth parts, the third maxillipeds, play little active part other than helping to retain sediment and water during the sorting process. The inner surface of the second maxilIipedries carries quite large numbers of long setae, some with spoon tips and others feathery. Facing these on the outer surface of the first maxillipeds is a brush of stiff setae. The sediment is rolled between two maxillipeds. The spooned setae of the second maxillipeds hold sand grains against the brushlike setae of the first maxillipeds, and diatoms and bacteria adhering to the grains are brushed off and moved towards the mouth itself. While this is going on, water is pumped out of the gill chamber into the buccal chamber. This helps the sorting process which takes place essentially in suspension…The mouth parts, too, are adapted to particular sediment compositions. Species feeding predominantly on coarse sandy sediments have more of the long, spoon-tipped setae on the inside of the second maxillipeds, and the tips of the setae are more spoon-shaped, while the setal 'brush' on the outside of the first maxillipeds is denser. Where the preferred sediment contains more fine organic particles, extra rows of setae are present at the base of the third maxillipeds to protect the aperture into the gill chamber and prevent the gills from becoming clogged. (Macnae 1968; Miller 1961; Ono 1965)." (Hogarth 1999:94-95)
Learn more about this functional adaptation.
Molecular Biology and Genetics
Statistics of barcoding coverage
Specimens with Sequences:434
Specimens with Barcodes:424
Species With Barcodes:43
- "Uca" redirects here. For other use, see UCA (disambiguation).
A fiddler crab, sometimes known as a calling crab, may be any of approximately 100 species of semi-terrestrial marine crabs which make up the genus Uca. As members of the family Ocypodidae, fiddler crabs are most closely related to the ghost crabs of the genus Ocypode. This entire group is composed of small crabs – the largest being slightly over two inches across. Fiddler crabs are found along sea beaches and brackish inter-tidal mud flats, lagoons and swamps. Fiddler crabs are most well known for their sexually dimorphic claws; the males’ major claw is much larger than the minor claw while the females’ claws are both the same size.
Like all crabs, fiddler crabs shed their shells as they grow. If they have lost legs or claws during their present growth cycle, a new one will be present when they molt. If the large fiddle claw is lost, males will develop one on the opposite side after their next molt. Newly molted crabs are very vulnerable because of their soft shells. They are reclusive and hide until the new shell hardens.
Found in mangroves, in salt marshes, and on sandy or muddy beaches of West Africa, the Western Atlantic, the Eastern Pacific and the Indo-Pacific, fiddler crabs are easily recognized by their distinctively asymmetric claws.
Fiddler crabs communicate by a sequence of waves and gestures; males have an oversized claw or chela; used in clashes of ritualised combat of courtship over a female and signal their intentions between conspecifics. The movement of the smaller claw from ground to mouth during feeding explains the crabs' common name; it looks as if the animal were playing the larger claw like a fiddle.
The crab's smaller claw picks up a chunk of sediment from the ground and brings it to the mouth, where its contents are sifted through (making the crab a detritivore). After anything edible is salvaged, be it algae, microbes, fungus, or other decaying detritus, the sediment is replaced in the form of a little ball. The presence of these sediment balls near the entrance to a burrow is a good indication of its occupation. Some experts believe that the feeding habits of fiddler crabs play a vital role in the preservation of wetland environments; by sifting through the sands, they aerate the substrate and prevent anaerobic conditions.
Fiddler crabs live rather brief lives of no more than two years (up to three years in captivity). Male fiddler crabs use the major claw to perform a waving display as a form of female courtship. Females choose their mate based on claw size and also quality of the waving display. In many fiddler crab species, the female occupies the burrow of their mate while she lays her clutch of eggs. Research shows that the male major claw size is also correlated with burrow width; the width of the burrow influences incubation temperature. Therefore, the female will choose a male mate whose claw size indicates the best burrow environment for her clutch of eggs. The waving display is also thought to indicate to females the overall healthiness of the male; a more vigorous display is more difficult to do and thus requires the male to be in prime health condition, which suggests that the male will help produce viable offspring.
Male versus male competition also occurs as a fighting with the major claws. If a male loses his larger claw, the smaller one will begin to grow larger and the lost claw will regenerate into a new (small) claw. For at least some species of fiddler crabs, however, the small claw remains small, while the larger claw regenerates over a period of several molts, being about half its former size after the first molt. The female fiddler carries her eggs in a mass on the underside of her body. She remains in her burrow during a two week gestation period, after which she ventures out to release her eggs into the receding tide. The larvae remain planktonic for a further two weeks.
Fiddler crabs such as Uca mjoebergi have been shown to bluff about their fighting ability. Upon regrowing a lost claw, a crab will occasionally regrow a weaker claw that nevertheless intimidates crabs with smaller but stronger claws. This is an example of dishonest signalling.
The dual functionality of the major claw of fiddler crabs has presented an evolutionary conundrum in that the claw mechanics best suited for fighting do not match up with the mechanics best suited for a waving display.
Subgenera and species
Fiddler crabs are occasionally kept as pets. The fiddler crabs sold in pet stores generally come from brackish water lagoons. Because they live in lower salinity water, pet stores may call them fresh-water crabs, but they cannot survive indefinitely in fresh water.
- M. S. Rosenberg (2001). "The systematics and taxonomy of fiddler crabs: a phylogeny of the genus Uca" (PDF). Journal of Crustacean Biology 21 (3): 839–869. doi:10.1651/0278-0372(2001)021[0839:TSATOF]2.0.CO;2. ISSN 0278-0372.
- Levinton, J. S., Judge, M. L., and Kurdziel, J. P., 1995, Functional differences between the major and minor claws of fiddler crabs (Uca, family Ocypodidae, order Decapoda, Subphylum Crustacea): A result of selection or developmental constraint?: Journal of Experimental Marine Biology and Ecology, v. 193, p. 147-160.
- M. J. How, J. M. Hemmi, J. Zeil & R. Peters (2008). "Claw waving display changes with receiver distance in fiddler crabs, Uca perplexa". Animal Behaviour 75 (3): 1015–1022. doi:10.1016/j.anbehav.2007.09.004.
- Pope, D. S., 2000, Testing function of fiddler crab claw waving by manipulating social context: Behavioral Ecology and Sociobiology, v. 47, p. 432-437.
- Perez, D. M., Rosenberg, M. S., and Pie, M. R., 2012, The evolution of waving displays in fiddler crabs (Uca spp., Crustacea: Ocypodidae): Biological Journal of the Linnean Society, v. 106, p. 307-315.
- Reaney, L. T., and Backwell, P. R. Y., 2007, Temporal constraints and female preference for burrow width in the fiddler crab, Uca mjoebergi: Behavioral Ecology and Sociobiology, v. 61, p. 1515-1521.
- Matsumasa, M., Murai, M., and Christy, J. H., 2013, A low-cost sexual ornament reliably signals male condition in the fiddler crab Uca beebei: Animal Behaviour, v. 85, p. 1335-1341.
- Callander, S., Kahn, A. T., Maricic, T., Jennions, M. D., and Backwell, P. R. Y., 2013, Weapons or mating signals? Claw shape and mate choice in a fiddler crab: Behavioral Ecology and Sociobiology, v. 67, p. 1163-1167.
- British Ecological Society (November 13, 2008). "Fiddler crabs reveal honesty is not always the best policy". University of New South Wales. Retrieved November 19, 2008.
- Simon P. Lailvaux, Leeann T. Reaney & Patricia R. Y. Backwell (November 11, 2008). "Regenerated claws dishonestly signal performance and fighting ability in the fiddler crab Uca mjoebergi". Functional Ecology 23 (2): 359. doi:10.1111/j.1365-2435.2008.01501.x. Retrieved November 18, 2008.
- Swanson, B. O., George, M. N., Anderson, S. P., and Christy, J. H., 2013, Evolutionary variation in the mechanics of fiddler crab claws: Bmc Evolutionary Biology, v. 13.
- Peter K. L. Ng, Danièle Guinot & Peter J. F. Davie (2008). "Systema Brachyurorum: Part I. An annotated checklist of extant Brachyuran crabs of the world" (PDF). Raffles Bulletin of Zoology 17: 1–286.
- Hsi-Te Shih, Tohru Naruse & Peter K. L. Ng (2010). "Uca jocelynae sp. nov., a new species of fiddler crab (Crustacea: Brachyura: Ocypodidae) from the Western Pacific" (PDF). Zootaxa 2337: 47–62.
- Lianne McLeod. "How to Set Up a Tank for Fiddler Crabs". About.com. Retrieved January 13, 2010.
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